The present invention is directed to novel methods for producing elastomer composite blends and to novel elastomer composite blends produced using such methods. More particularly, the invention is directed to methods for producing elastomer composite blends having particulate filler finely dispersed in elastomer, and to elastomer composite blends such as curative-free compositions, curative-bearing compositions, and vulcanized rubber materials and products formed of such compositions.
Numerous products of commercial significance are formed of elastomeric compositions wherein particulate filler is dispersed in any of various synthetic elastomers, natural rubber or elastomer blends. Carbon black, for example, is widely used as a reinforcing agent in natural rubber and other elastomers. It is common to produce a masterbatch, that is, a premixture of filler, elastomer and various optional additives, such as extender oil, and then in some cases to blend such masterbatch with additional elastomer in a subsequent mixing step. Carbon black masterbatch is prepared with different grades of commercially available carbon black which vary both in surface area per unit weight and in xe2x80x9cstructure.xe2x80x9d Numerous products of commercial significance are formed of such elastomeric compositions of carbon black particulate filler dispersed in natural rubber. Such products include, for example, vehicle tires wherein different elastomeric compositions may be used for the tread portion, sidewalls, wire skim and carcass. Other products include, for example, engine mount bushings, conveyor belts, windshield wipers and the like. While a wide range of performance characteristics can be achieved employing currently available materials and manufacturing techniques, there has been a long standing need in the industry to develop elastomeric compositions having improved properties and to reduce the cost and complexity of current manufacturing techniques. In particular, it is known for example that macro-dispersion level, that is, the uniformity of dispersion of the carbon black or other filler within the elastomer, can significantly impact performance characteristics. For elastomeric compositions prepared by intensively mixing the carbon black or other filler with natural rubber or other elastomer (such as in a Banbury mixer or the like), any increase in macro-dispersion requires longer or more intensive mixing, with the consequent disadvantages of increased energy costs, manufacturing time, and similar concerns. For carbon black fillers of certain surface area and structure characteristics, dispersion beyond a certain degree has not been possible or commercially practicable using known mixing apparatus and techniques. In addition, such prolonged or more intensive mixing degrades the natural rubber by disruption of the polymeric chains of the natural rubber elastomer, and so reduces its molecular weight, rendering the finished elastomeric compound undesirable for certain applications. For use in tire tread, for example, reduced molecular weight is known to cause an undesirable increase in the so-called rolling resistance of the tire.
It is well known to employ carbon blacks having higher or lower structure and surface area to manipulate the performance characteristics of an elastomeric composition. Carbon blacks of higher surface area and lower structure are known to improve crack growth resistance and cut-and-chip resistance as well as, generally, abrasion resistance, and other performance qualities. Commercially available mixing techniques have been unable to achieve excellent uniformity of dispersion of carbon blacks throughout the elastomer, however, without unacceptable degradation of the natural rubber. In fact, for typical carbon black loading levels in natural rubber, such as 45 phr to 75 phr, and oil loading from 0 phr to 10 phr, low structure carbon blacks, such as carbon blacks of DBPA less than 110 cc/100 g, particularly those having surface area above about 45 m2/g to 65 m2/g (CTAB), it has not been possible to achieve compounds having less than about 1% undispersed carbon black (measured as macro-dispersion, as described below) regardless of the duration and intensity level of mixing.
Furthermore, while theoretical analysis has indicated desirable improvements in certain performance characteristics of elastomeric compositions employing carbon blacks of higher surface area and lower structure, it has not been possible using known physical milling or other mastication processes to obtain such elastomeric compositions in which both the molecular weight of the natural rubber is well preserved and satisfactory macro-dispersion levels of the carbon black are achieved. Generally, it has been found, for example, that the elastomer reinforcing properties of a carbon black increase as the particle size of the carbon black decreases. However, with extremely fine carbon blacks an anomalous condition is known to be encountered, in which the expected improvement in properties is not achieved. This is understood to be due at least in part to the inability of conventional elastomer compounding methods to adequately disperse the carbon black in the natural rubber without undue breakdown of the elastomer polymer. There has been, therefore, consequent inability to take full advantage of the natural affinity of the carbon black and the natural rubber for each other in the case of such carbon blacks.
Since good dispersion of carbon black in natural rubber compounds has been recognized for some time as one of the most important objectives for achieving good quality and consistent product performance, considerable effort has been devoted to the development of procedures for assessing dispersion quality in rubber. Methods developed include, e.g. the Cabot Dispersion Chart and various image analysis procedures. Dispersion quality can be defined as the state of mixing achieved. An ideal dispersion of carbon black is the state in which the carbon black agglomerates (or pellets) are broken down into aggregates (accomplished by dispersive mixing) uniformly separated from each other (accomplished by distributive mixing), with the surfaces of all the carbon black aggregates completely wetted by the rubber matrix (usually referred to as incorporation).
Common problems in the rubber industry which are often related to poor macro-dispersion can be classified into four major categories: product performance, surface defects, surface appearance and dispersion efficiency. The functional performance and durability of a carbon black-containing rubber formulation, such as tensile strength, fatigue life and wear resistance, are affected substantially by macro-dispersion quality. Undispersed carbon black can also cause surface defects on finished products, including visible defects. Eliminating the presence of surface defects is of critical importance in molded thin parts for functional reasons and in extruded profiles for both aesthetic and functional reasons.
A commercial image analyzer such as the IBAS Compact model image analyzer available from Kontron Electronik GmbH (Munich, Germany) can be used to measure macro-dispersion of carbon black or other filler. Typically, in quantitative macro-dispersion tests used in the rubber industry, the critical cut-off size is 10 microns. Defects larger than about 10 microns in size typically consist of undispersed black or other filler, as well as any grit or other contaminants, which can affect both visual and functional performance. Thus, measuring macro-dispersion involves measuring defects on a surface (generated by microtoming, extrusion or cutting) greater than 10 microns in size by total area of such defects per unit area examined using an image analysis procedure. Macro-dispersion D(%) is calculated as follows:
      %    ⁢          xe2x80x83        ⁢    Undispersed    ⁢          xe2x80x83        ⁢    area    ⁢          xe2x80x83        ⁢          (      %      )        =            1              A        m              ⁢                  ∑                  i          =          1                m            ⁢                        N          i                ⁢                  xe2x80x83                ⁢                              π            ⁢                          xe2x80x83                        ⁢                          D              i              2                                4                    
where
Am=Total sample surface area examined
Ni=Number of defects with size Di 
Di=Diameter of circle having the same area as that of the defect (equivalent circle diameter).
m=number of images
Macro-dispersion of carbon black or other filler in uncured natural rubber or other suitable elastomer can be assessed using image analysis of cut surface samples. Typically, five to ten arbitrarily selected optical images are taken of the cut surface for image analysis. Knife marks and the like preferably are removed using a numerical filtering technique. Cut surface image analysis thus provides information regarding the carbon black dispersion quality inside a natural rubber compound. Specifically, percent undispersed area D(%) indicates carbon black macro-dispersion quality. As macro-dispersion quality is degraded, percent undispersed area increases. Dispersion quality can be improved, therefore, by reducing the percent undispersed area. As noted above, the mixing operations have a direct impact on mixing efficiency and on macro-dispersion. In general, better carbon black macro-dispersion is achieved in the elastomer, for example in a natural rubber masterbatch, by longer mixing and by more intensive mixing. Unfortunately, however, achieving better macro-dispersion by longer, more intensive mixing, degrades the elastomer into which the carbon black is being dispersed. This is especially problematic in the case of natural rubber, which is highly susceptible to mechanical/thermal degradation. Longer and more intensive mixing, using known mixing techniques and apparatus, such as a Banbury mixer, reduces the molecular weight of the natural rubber masterbatch-composition. Thus, improved macro-dispersion of carbon black in natural rubber is known to be achieved with a corresponding, generally undesirable reduction in the molecular weight of the rubber.
In addition to dry mixing techniques, it is known to continuously feed latex and a carbon black slurry to an agitated coagulation tank. Such xe2x80x9cwetxe2x80x9d techniques are used commonly with synthetic elastomer, such as SBR. The coagulation tank contains a coagulant such as salt or an aqueous acid solution typically having a pH of about 2.5 to 4. The latex and carbon black slurry are mixed and coagulated in the coagulation tank into small beads (typically a few millimeters in diameter) referred to as wet crumb. The crumb and acid effluent are separated, typically by means of a vibrating shaker screen or the like. The crumb is then dumped into a second agitated tank where it is washed to achieve a neutral or near neutral pH. Thereafter the crumb is subjected to additional vibrating screen and drying steps and the like. Variations on this method have been suggested for the coagulation of natural and synthetic elastomers. In U.S. Pat. No. 4,029,633 to Hagopian et al, which like the present invention is assigned to Cabot Corporation, a continuous process for the preparation of elastomer masterbatch is described. An aqueous slurry of carbon black is prepared and mixed with a natural or synthetic elastomer latex. This mixture undergoes a so-called creaming operation, optionally using any of various known creaming agents. Following the creaming of the carbon black/latex mixture, it is subjected to a coagulation step. Specifically, the creamed carbon black/latex mixture is introduced as a single coherent stream into the core of a stream of coagulating liquor. The solid stream of creamed carbon black/latex mixture is said to undergo shearing and atomizing by the stream of coagulating liquor prior to coagulation, being then passed to a suitable reaction zone for completion of the coagulation. Following such coagulation step, the remainder of the process is substantially conventional, involving separation of the crumb from the waste product xe2x80x9cserumxe2x80x9d and washing and drying of the crumb. A somewhat similar process is described in U.S. Pat. No. 3,048,559 to Heller et al. An aqueous slurry of carbon black is continuously blended with a stream of natural or synthetic elastomer or latex. The two streams are mixed under conditions described as involving violent hydraulic turbulence and impact. As in the case of the Hagopian et al patent mentioned above, the combined stream of carbon black slurry and elastomer latex is subsequently coagulated by the addition of an acid or salt coagulant solution.
There has long been a need in various industries for elastomeric compounds of particulate filler dispersed in suitable elastomer having improved macro-dispersion, especially, for example, carbon black dispersed in natural rubber blended with another elastomer. As discussed above, improved macro-dispersion can provide correspondingly improved aesthetic and functional characteristics. Especially desirable are new elastomeric compounds of carbon black in a blend of natural rubber and synthetic elastomer, wherein improved macro-dispersion is achieved together with higher molecular weight of the natural rubber. It is an object of the present invention to meet some or all of these long felt needs.
In accordance with a first aspect, a method for preparing elastomer composite blends comprises first preparing elastomer masterbatch by feeding simultaneously a particulate filler fluid and an elastomer latex fluid to a mixing zone of a coagulum reactor. Preferably, the coagulum reactor has an elongate coagulum zone extends from the mixing zone, most preferably having a progressively increasing cross-sectional area in the downstream direction toward a discharge end of the coagulum reactor. The elastomer latex may be either natural or synthetic and the particulate filler fluid comprises carbon black or other particulate filler effective to coagulate the latex. The particulate filler fluid is fed to the mixing zone preferably as a continuous, high velocity jet of injected fluid, while the latex fluid is fed at low velocity. The velocity, flow rate and particulate concentration of the particulate filler fluid are sufficient to cause mixture with high shear of the latex fluid and flow turbulence of the mixture within at least an upstream portion of the coagulum zone so as to substantially completely coagulate the elastomer latex with the particulate filler prior to the discharge end. Substantially complete coagulation can thus be achieved, in accordance with preferred embodiments, without the need of employing an acid or salt coagulation agent. The coagulated product of such wet mixing step then is dry mixed with additional elastomer to form an elastomer composite blend. Such additional elastomer may be the same as, or different from, the elastomer used in the wet mixing step. Optionally, additional filler can be added during the dry mixing step. Such additional filler can be the same as, or different from, the particulate filler used in the wet mixing step.
In accordance with yet another aspect, elastomer composite blends are provided as a product of the process disclosed here. In accordance with preferred embodiments, novel elastomer composite blends are provided having macro-dispersion level of the particulate filler, molecular weight of the elastomer, particulate loading level, choice of particulate filler (including, for example, carbon black fillers of exceptionally high surface area and low structure) and/or other characteristics not previously achieved. In that regard, preferred elastomer composite blends disclosed here have excellent macro-dispersion, even of certain fillers, such as carbon blacks having a structure to surface area ratio DBP:CTAB less than 1.2 and even less than 1, in elastomers such as natural rubber, etc. with little or no degradation of the molecular weight of the elastomer. In accordance with yet other aspects of the invention, intermediate products are provided as well as final products which are formed of the elastomer composite blends produced by the method disclosed here. Macro-dispersion here means the macro-dispersion D(%) of the particulate filler measured as percent undispersed area for defects larger than 10 microns. In elastomer composite blends disclosed here comprising natural rubber, the molecular weight of the natural rubber, that is, the MWsol (weight average) of the sol portion, preferably is at least about 300,000, more preferably at least about 400,000, being in certain preferred embodiments between 400,000 and 900,000. The elastomer composite blends optionally comprise extender oil, such as about 0 to 20 phr, more preferably about 0 to 10 phr extender oil, and/or other ingredients such as are well known for optional use in compounding natural rubber and/or other elastomers with carbon black and/or other fillers. As discussed further below in connection with certain preferred and exemplary embodiments, the novel elastomer composite blends disclosed here can provide highly desirable physical properties and performance characteristics. Accordingly, the invention presents a significant technological advance.
These and other aspects and advantages of various embodiments of the invention will be further understood in view of the following detailed discussion of certain preferred embodiments.